FERMILPSC - Fermi LAT 8-Year Point Source Catalog

The Fermi LAT 8-Year Point Source Catalog (4FGL) is a catalog of high-energy
gamma-ray sources detected by the Large Area Telescope (LAT) on the Fermi
Gamma-ray Space Telescope (Fermi) mission during the first eight years of
science data.

Relative to the 3FGL (4-year) catalog, the 4FGL catalog has twice as much
exposure as well as a number of analysis improvements, including an updated
model for Galactic diffuse gamma-ray emission. The 4FGL catalog includes 5065
sources above 4 sigma significance, for which we provide localization and
spectral properties. Seventy-five sources are modeled explicitly as spatially
extended, and overall 355 sources are considered as identified based on angular
extent or correlated variability observed at other wavelengths. For 1323 sources
we have not found plausible counterparts at other wavelengths. More than 3130 of
the identified or associated sources are active galaxies of the blazar class,
239 are pulsars.

We emphasize several important points and caveats regarding use of the 4FGL catalog:

This work is a summary of the LAT results covering the time
period August 4, 2008, to August 2, 2016.

The catalog is based on analysis of Pass 8 data in the 50 MeV to 1 TeV energy range.

Source detection is based on the integrated data set, i.e., sources are detected according to their average fluxes.

The catalog adopted multiple spectral shapes, depending on the observed source properties. Pulsars are modeled by an exponentially cutoff Power Law. Others are modeled as a LogParabola if statistically significant curvature (according to Signif_Curve) is detected, or as a simple Power Law otherwise.

After computing the maximum likelihood, the threshold for inclusion is likelihood Test Statistic TS > 25, corresponding to somewhat more than 4 sigma significance. The catalog has 5098 sources.

The corresponding energy flux limit varies according to the
hardness of the source spectrum and the brightness of the background Galactic diffuse emission.

Catalog entries include quality flags that indicate potential issues: any non-zero entry in
the "Flags" column indicates that some inconsistency was seen during the analysis (see
the table in the preprint for details). Sources with these indicators
should be used with great care. They correspond to significant excesses of photons, but such excesses can result from residual extended emission or confused source
pile-up.

The catalog analysis assumed point source emission for all but seventy-five
extended sources, which were modeled using spatial templates and are indicated by an "e" following the name. Results for extended sources such as the LMC and Cen A are likely
to not be representative of the total emission, and additional catalog point sources may appear near such objects.

The Crab pulsar and PWN are represented by a total of three entries, two of which (designated "i" for inverse compton and "s" for synchrotron) represent spectral
components of the PWN. We consider these three entries to represent two sources.

Associations listed in the catalog are not in general to be taken as firm identifications. Associations are positional coincidences that are statistically unlikely to be due to chance alignment between known or candidate gamma-ray producing objects
with 4FGL sources. Upper case in the CLASS column means identified. Except for those cases with correlated variability or spatial extent, a physical relationship is not established. The designators for some
associated sources may be updated in a future release to use more common or more complete names.

Name
The source designation, 4FGL JHHMM.m+DDMM[c], constructed according to the IAU
Specifications for Nomenclature, in which the Right Ascension and Declination
have been truncated to 0.1 decimal minutes and 1', respectively. The '4' refers
to the eight-year catalog (the first-year catalog was '1'; the second-year
catalog was '2'; the four-year catalog was '3') and 'FGL' represents Fermi
Gamma-ray LAT. The letter at the end can be "c" (coincident with interstellar
clump), "e" (extended source), "i" (for Crab nebula inverse Compton) or "s" (for
Crab nebula synchrotron).

RA
The Right Ascension of the gamma-ray source in the selected equinox.
This was given in J2000 decimal degree coordinates in the original table.

Dec
The Declination of the gamma-ray source in the selected equinox.
This was given in J2000 decimal degree coordinates in the original table.

LII
The Galactic Longitude of the gamma-ray source, as derived from the
J2000 position in the original table.

BII
The Galactic Latitude of the gamma-ray source, as derived from the
J2000 position in the original table.

Semi_Major_Axis_68
The semi-major axis of the error ellipse at 68% confidence, in degrees.

Semi_Minor_Axis_68
The semi-minor axis of the error ellipse at 68% confidence, in degrees.

Position_Angle_68
The position angle of the 68%-confidence semi-major axis, from celestial
North, positive toward increasing R.A. (eastward), in degrees.

Semi_Major_Axis_95
The semi-major axis of the error ellipse at 95% confidence, in degrees.

Semi_Minor_Axis_95
The semi-minor axis of the error ellipse at 95% confidence, in degrees.

Position_Angle_95
The position angle of the 95%-confidence semi-major axis, from celestial
North, positive toward increasing R.A. (eastward), in degrees.

Pivot_Energy
The energy, in MeV, at which the error in the differential photon flux is
minimal (i.e., the decorrelation energy for the power-law fit). This is
derived from the likelihood analysis for 100 MeV - 1 TeV.

Variability_Index
The sum of the log(likelihood) difference between the flux fitted in each
time interval and the average flux over the full catalog interval; a value
greater than 18.48 over 12 intervals indicates <1% chance of being a steady
source. See the accompanying paper for details.

Frac_Variability
The fractional variability computed from the fluxes in each year.

TeVCat_Assoc
The designation of the likely corresponding source from the TeVCat catalog, if
any.

Source_Type
The primary class designation of the identification or likely associated
source. Refer to the accompanying paper for details of class designations and
the association method. Note that designations shown in capital letters are
firm identifications; lower case letters indicate associations. The following
abbreviations are used:

Assoc_Name
The designation of the identified or likely associated source.

Assoc_Name_Alt
The alternate designation or an indicator as to whether the source is inside
an extended source.

Assoc_Prob_Bay
The probability of association according to the Bayesian method. It is set to
NULL for extended sources (which do not enter that calculation) and to 0 for
point sources that did not reach 0.8, the threshold for declaring associations.

Assoc_Prob_LR
The probability of association according to the Likelihood Ratio method. It
is set to NULL for extended sources (which do not enter that calculation) and
to 0 for point sources that did not reach 0.8, the threshold for declaring
associations.

Assoc_Error_Radius
The position uncertainy at 68% confidence level of the counterpart
localization for assoc_name, in degrees.

Analysis_Flags
These analysis flags indicate possible issues noted in the detection or
characterization of the source. A value of 0 indicates no known problem. Each
condition present raises a unique bit flag. The flag values are bit-encoded
into a single integer, with flag N contributing 2N-1 to the value, where N
is given as:

N=1: Source with TS > 35 which went to TS < 25 when changing the diffuse
model. Note that sources with TS < 35 are not flagged with this bit
because normal statistical fluctuations can push them to TS < 25.
N=2: Not used.
N=3: Flux (> 1 GeV) or energy flux (> 100 MeV) changed by more than 3
sigma when changing the diffuse model or the analysis method. Requires
also that the flux change by more than 35% (to not flag strong
sources).
N=4: Not used.
N=5: Not used.
N=6: Not used.
N=7: Not used.
N=8: Not used.
N=9: Localization Quality > 8 in pointlike (see Section 3.1 in catalog
paper) or long axis of 95% ellipse > 0.25.
N=10: Spectral Fit Quality > 30 in pointlike.
N=11: Not used.
N=12: Highly curved spectrum; LogParabola beta fixed to 1 or PLEC_Index
fixed to 0 (see Section 3.3 in catalog paper).